text: In 1828, when he already turned 32, Claus decided to continue his education at the University of Tartu. During the course of his studies, in 1831, he started working as an assistant at the chemical laboratory of the university. He graduated in 1835, and in 1837, defended his PhD thesis on analytical phytochemistry ("Grundzüge der Analytischen Phytochemie") at the University of Tartu. In the same year, he applied at the St. Petersburg Medical-Surgical Academy for an academic position and obtained a post of the head of chemical laboratory at Kazan State University. In 1839, he defended habilitation thesis on separation of minerals in mineral waters and was appointed an assistant professor. He became full professor in chemistry in 1844.